People’s assessments of who won the debate — and their evaluations of the candidates more generally — were affected by which media outlet they were assigned to watch after the debate. The favorability of the coverage changed voters’ minds about the debate they had just seen only moments before.

Even interested and attentive voters are swayed by how reporters frame and summarize events.

The takeaway? The candidates’ performances tonight are important (as are their policy positions, of course), but the news coverage of the debate might matter more than the actual event.

Lynn Vavreck, a professor of political science at U.C.L.A., is a co-author of “The Gamble,” about the 2012 presidential campaign. Follow her on Twitter at @vavreck.